6 replies to this topic

[dhns]

Dear all ,

Please explain anyone,

the term Compressibility factor " Z<0 " is possible for real field application,

In what condition it will be occur ?!

Regards,
Dhns.

[smalawi]

Hi,

for a liquid stream Z can be very small but always positive, of course cant say its meaningful for liquids.

for gases its around 1, have a look at the compressibility charts

can you let us know when its negative ?

cheers,

sm

[Zauberberg]

Dhns,

Please take a look at gas equation: pV = zRT
Since all factors contributing in this equation are positive (pressure, volume, temperature, and gas constant), how would you explain the possibility of compressibility factor Z to be negative?

Is it a "real field application" where absolute pressure, absolute temperature or specific volume can become negative?
Well yes, if you operate the system at vacuum conditions and you read negative pressure on the gauge meter!

[Qalander (Chem)]

Dear Friend Zauberberg

Hello/Good Afternoon,

Nice Explanation indeed!

[dhns]

Mr.Zauberberg,

Thanks for your Explanatory notes ,
your notes also mentioning the same as of my clouds

Even though, If my system is operating under negative temperature (like -100 Deg C) ,
or Negative Pressure (Your also mentioned in your talk ) (is Absolute vacuum is covering the scale range below 0 kPa (a). (ie.-101.325), is there any pressure term like that below abs.vacuum in today's industrial life? If it is, why is not possible to have Z < 0 ?
Please Clarify for me.

Kind Thanks,
dhns

[ankur2061]

dhns,

In the ideal gas equation - or any equation of state - pressure and temperature are absolute values. In either SI units or english units, the absolute pressure cannot be less than 0 kPa (abs) / 0 psia which is also equivalent to Full Vacuum. Similarly, the absolute temperature cannot be less than 0 K / 0 R. This implies that when you calculate the compressibility factor Z it cannot be computed as a negative value.

This is very fundamental & I expect that this would be known to even a first year engineering student.

Regards,
Ankur.

[Zauberberg]

Always be sure what kind of measurement(s) and values you use in your calculations, and always try to encompass your work with common engineering sense, like it has been demonstrated above. Without this common sense "tool", given by birth to all of us, even the most sophisticated tools like simulation software can turn your "work" into a bunch of garbage. Remember - engineers used to design complex process plants even before the simulation software has been invented. So how did they do that?